Which one of the following statements about the control of enzyme activity by phosphorylation is correct?
(a) Phosphorylation of an enzyme results in a conformational change
(b) Phosphorylation of an enzyme occurs only at specific tyrosine residues
(c) Phosphorylation of an enzyme is carried out by phosphoprotein phosphatases
(d) Enzyme control by phosphorylation is irreversible
